Recently, progresses have been made of digital transmission systems in which various kinds of information such as video, audio, and text data are multiplexed for transmission. For example, in ISDB (Integrated Services Digital Broadcasting) and the like, flexible service organization has been under way to address the diversification of services, aiming at integrated digital broadcasting where all information is handled as digital data.
In the meantime, digital receiving apparatuses for receiving transmission signals transmitted from broadcast stations have implemented measures that take account of reception quality. For example, digital receiving apparatuses mounted on mobile units such as an automobile have implemented measures for providing useful information to users without interruption even when the field intensity varies while moving due to the effect of buildings and the like, and when receiving incoming waves that cause various kinds of fading.
A digital broadcast receiving apparatus disclosed in Japanese Patent Application Laid-Open No. 2003-274302 receives radio waves of a multicarrier modulation transmission method transmitted from a broadcast station, and selects and demodulates a modulation signal that is modulated by a modulation method of higher error tolerance when undergoing the foregoing changes in field intensity or receiving incoming waves that cause various types of fading. This makes it possible to provide useful information to users without interruption.
More specifically, the broadcast station modulates broadcast programs by modulation methods having different error tolerances (such as DQPSK, QPSK, 16QAM, and 64QAM), assigns the same to a plurality of narrowband channels, and transmits them by a multicarrier transmission method for broadband transmission. When receiving the incoming waves that vary in field intensity and/or cause various kinds of fading, the digital broadcast receiving apparatus selects and modulates a modulation signal that is modulated by a modulation method of favorable error tolerance. This makes it possible to provide useful information to users without interruption.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2003-274302